1. Field of the Invention
The present invention relates to an X-ray generation apparatus, specifically, one which makes it possible to generate high X-ray output by use of a smaller apparatus than the conventional size apparatus.
The ordinary method, which generates X-rays using irradiation of accelerated electrons to a target, adapted an X-ray generation apparatus. However, when electrons, which are accelerated by some tens of thousands voltage, collide with the target, only 1% of the accelerated electron energy changes to X-ray energy and the remaining 99% is consumed by Joule's heat. It is essential to investigate how to effectively radiate one hundred times the thermal energy incidental to X-ray generation from the target, in order to obtain a high output X-ray generation apparatus. The range of X-ray strength generated by an apparatus depends on the target material and cooling ability. The generated X-ray energy can be increased by increasing electron irradiation energy within a range of the target not melted by irradiation of accelerated electrons.
Therefore, metal materials which have high thermal conductivity and high melting temperatures are mainly used as the X-ray target, and the thermal energy is radiated by water cooling. Furthermore, in order to obtain high strength X-rays, a method by which the target is cooled while rotating has been developed. In this method, a portion of the target which is irradiated by electrons and emits X-rays, rotates one after another, the temperature of the target does not increase, and higher X-ray energy can be obtained compared with a fixed type target.
2. Description of the Prior Art
A diamond containing target, in which the diamond is embedded in a copper substrate by powder sintering, is used and the target is cooled and rotated in an X-ray generation apparatus shown in Tokkai-Sho 57 (1982)-38548. However, it has been pointed out that as the size of such X-ray apparatus increases, it is imperative to prevent vibration when rotating the target. Furthermore, there are problems with decreased efficiency of the electron beam when the electron beam irradiates both copper and diamond.
An X-ray generation apparatus, in which an electron beam irradiates in the direction of a heat resistant single crystal axis, emits X-rays in the direction of the single crystal axis and a cooling means of the single crystal is prepared, as shown in Tokkai-Hei 2 (1990)-309596. However, the target is cooled insufficiently because the electron irradiating portion of the target is cooled through the peripheral portion of the single crystal.
An anticathode for X-ray generation which is made from a 2-layer structure of high heat conductive inorganic material and thin metal film, is shown in Tokkai-Hei 5 (1993)-343193. Effective cooling is expected when the back portion of the high heat conductive inorganic material is cooled as shown in this prior art. However, when the target is adapted for an X-ray generation apparatus and is cooled at the peripheral portion (as shown in Tokkai-Hei 2-309596), the target does not have sufficient cooling ability because a considerable amount of thermal energy diffuses along the thin metal film for which heat conduction is rather high. The other problem is exfoliation of the thin metal film. A method of synthesizing diamond from the gaseous phase is disclosed in U.S. Pat. No. 4,767,608 issued Aug. 30, 1988, and in U.S. Pat. No. 4,434,188 issued Feb. 28, 1994.
Spitsyn, U.S. Pat. No. 5,148,462, discloses a diamond substrate having a linear-shaped target made from a groove filled with target material. The linear shape target lacks the advantages of the target in the present invention made from a hole filled with target material of high cooling efficiency. Specifically, when the direction of the electron beam coincides with the direction of the penetration of the target, as in the hole configuration of the claimed invention, the electron beam reaches the inner portion of the target and the absorption ratio of the electron beam increases. The increased absorption results in an increased X-ray output.
Further, Spitsyn's device has a surface coating on the electron impinging side of the device, while the metal film or electric conductive diamond layer of the present invention is located on the side of the substrate that is not impinged with electrons (i.e. the back surface). Spitsyn's invention does not contemplate adding a layer to the back surface as in the present invention. Although Spitsyn discloses a surface coating, this coating is on the electron impinging side, and it generates some X-rays in the surface coating. The metal film or the electric conducting diamond layer, in combination with the hole configuration in the present invention, prevents X-ray formation in this layer.
The use of cooling holders is known in the art. Cooling an anticathode using such known holder merely cools the peripheral portion of the anticathode. The holder, therefore, inefficiently cools the entire anticathode because the cooling means is not proximate the target, which is the source of heat generation when electrons collide with the target. The present invention includes a much more efficient way to cool the targets. Cooling passages formed inside the anticathode itself surround the target, not merely the peripheral portion of the anticathode. Thus, more efficient cooling of the anticathode is possible. In an apparatus such as Spitsyn, it would be difficult to employ such cooling passages or tubes inside the anticathode because the groove-type target of Spitsyn would interfere with a practical pathway to pass coolant. It would be economically impractical to increase the size of an expensive diamond substrate simply to accommodate the configuration of a groove-type structure such as the one in Spitsyn. Using a hole configuration as in the present invention, the diamond substrate can remain smaller, and thus less expensive, and at the same time contain an efficient internal cooling means.